This invention relates to a resistance heated flash evaporator for the vaporization of metals and more particularly to a flash evaporator vaporization boat composed of a graphite body and a pyrolytic boron nitride outer coating.
Vacuum deposition is a common method for coating metals such as aluminum, copper, zinc, and tin onto various substrates of metal, glass and plastic. The metal is vaporized in a vessel commonly referred to as a xe2x80x9cvaporization boatxe2x80x9d by electric resistance heating. The boat is connected to a source of electrical power to heat the vessel to a temperature which will cause the metal charge in contact with the boat to vaporize. Typically, the product is placed in an evacuated chamber within which the metal is vaporized. The product may be fed individually or continuously into the chamber or, alternatively, may form part of the chamber itself. A wide variety of product is coated with metal using vapor deposition including ,e.g., television picture tubes, automobile headlights, toys and the like.
The metal charge is placed in a cavity recess machined into the top surface of the boat. Upon reaching the vaporization temperature of the metal the metal charge melts and quickly vaporizes. The charge flashes in a vigorous and abrupt manner and the process is repeated with a new charge of metal. The number of flashes which can be performed in a given time period controls product production. Accordingly, the number of flashes a boat can sustain without failure is a critical performance characteristic of the process. Conversely, when failure occurs the parts being coated are scrapped or have to be redone which involves additional time and cost. To avoid this extra cost the vaporization vessel may be discarded before anticipated failure. A vaporization boat with a short lifetime increases the production cost per part and reduces efficiency.
A method of resistance heating of metal using a flash evaporator composed of a pyrolytic boron nitride coated graphite body is taught in U.S. Pat. No. 5,239,612 the disclosure of which is herein incorporated by reference. As described in the aforementioned patent the coating of pyrolytic boron nitride (hereinafter xe2x80x9cPBNxe2x80x9d) may be formed by the vapor phase reaction of ammonia and a boron halide such as boron trichloride. The basic chemical vapor deposition process for forming pyrolytic boron nitride is taught in U.S. Pat. No. 3,152,006 and it is well known to those skilled in this art that vapor deposited PBN has a hexagonal crystal structure. It has been discovered in accordance with the present invention that the useful life of the vaporization boat may be extended by increasing the density of the PBN outer coating in contact with the metal to be vaporized to a density above at least 2.19 gm/cc and preferably between 2.19 gm/cc and 2.2 gm/cc.
The flash evaporator vaporization boat of the present invention comprises: a graphite body having a recessed cavity and an outer surface composition of high density pyrolytic boron nitride above a density of at least 2.19 gm/cc.